a) Field of the Invention
This invention relates to a music box in which pins inserted in a drum pluck teeth which vibrate to produce music.
b) Description of the Related Art
In a music box in which pins inserted in a drum pluck vibrating teeth to produce music, the drum rotation is kept constant by transmitting a driving force from a rotating driving source to a speed governor, via the drum. The governor is used for regulating the drum rotation, and is comprised of an accelerating train and a control means. There is known one such accelerating train in which a plurality of worms and worm wheels are provided in the train due to the associated advantage of having a large acceleration rate. For example, when worms are used in the middle of the accelerating train and in the last row, a brake member is provided in the worm used in the last row to rotate it at high speed, as friction resistance for regulating the drum rotation.
Also, a spring used as a driving source for the drum is wound around a winding shaft and is installed in the space called a spring housing constructed by a frame, case, and the like of the music box. The spring is arranged such that one end is inserted into the spring housing (from the outside of the housing) and engaged with the winding shaft projecting inside of the spring housing, and the other end thereof is engaged with an engagement section formed on the outer circumference of the spring housing, thus preventing the other end from entering in the spring housing while winding the spring. A tie section called necking is formed at the other end of the spring engaged with the engagement section. The tie section is formed with a gap created with the engagement section in the direction the spring extends, taking into account processability at the winding of the spring and at the engaging of the other end; the spring in the housing is tightened when being spirally wound around the winding shaft, and is gradually released by the operation of the speed governor to rotate the drum for the play.
In a music box, a worm is supported with allowance in the shaft direction (generally called a thrust allowance), taking into account the capability of assembling the worm and the tolerance of components. However, the worm is shifted by a deviation force generated in the axial direction when it is given a rotation from a worm wheel and rotated at high speed. The direction in which the deviation force is generated varies according to the input direction of the worm, i.e., the direction the worm rotates. For example, when the input to the worm is transmitted from the driving source side, the deviation force moves in the direction of input by placing tangential stress on the shaft; when the input from the driving source side is decreased and the inertial force of the brake, rotating at high speed, becomes large, the deviation force moves in the opposite direction to the above mentioned direction of input tangential stress.
When the moving direction of the worm varies due to the direction of the deviation force generated in such a way, the gap between the worm and the worm wheel becomes large when switching the directions of the deviation force; therefore, the brake does not function well, freeing the driving source and therefore rotating the drum radically. There would be no problem if the thrust allowance of each worm can be made zero, but it is virtually impossible because of tolerance of components and the like. Even if it could be made zero, a support section for supporting the worm is worn out by the deviation force applied on the worm in the direction the deviation force is applied with some material, thus generating the movement in the thrust direction.
The variation in the drum rotation due to a sudden rotation of the drum causes problems in deteriorating sound quality and making it difficult to hear the music. Specifically, when a plurality of worms are used in the accelerating train, the acceleration rate of the accelerating means is large; therefore, the axial movement of the worm provided in the middle of the train due to the deviation force greatly affects the engagement condition with the worm wheel on the deceleration side.
When a spring is used as a driving source for a drum, if the relationship between a first friction resistance (a friction resistance between the outermost peripheral surface of the spring and the inner peripheral surface of the spring housing) and a second friction resistance (a friction resistance between the outermost peripheral surface and a second outer peripheral surface, inner surface by one wind from the outermost) is broken during release of the spring, i.e., during the play, the spring is not released smoothly, and sometimes the edge engaged with the engagement section moves in the direction to push it out of the spring housing. Under such a condition, a clicking noise is generated between the edge of the spring and the engagement section, causing a noise during the play which is another problem while the music is played. On the other hand, if the relationship between the first and second friction resistance is always maintained such as "the first friction&gt;the second friction", the spring can be smoothly released, without causing a problem; however, it is difficult to maintain the friction resistance in the above mentioned relationship. To obtain such a relationship, many attempts are required, providing poor reliability, requiring a longer time to develop a product, and increasing the manufacturing cost.